Imaging post structures using x and y dipole optics and a single mask

ABSTRACT

A photolithographic method uses different exposure patterns. In one aspect, a photo-sensitive layer on a substrate is subject to a first exposure using optics having a first exposure pattern, such as an x-dipole pattern, followed by exposure using optics having a second exposure pattern, such as a y-dipole pattern, via the same mask, and with the photo-sensitive layer fixed relative to the mask. A  2 -D post pattern with a pitch of approximately 70-150 nm may be formed in a layer beneath the photo-sensitive layer using 157-193 nm UV light, and hyper-numerical aperture optics, in one approach. In another aspect, hard baking is performed after both of the first and second exposures to erase a memory effect of photoresist after the first exposure. In another aspect, etching of a hard mask beneath the photo-sensitive layer is performed after both of the first and second exposures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending, commonly assigned U.S.patent application Ser. No. 11/618,776, filed Dec. 30, 2006 (docket no.SAND-01183U50), published on Jul. 3, 2008 as US2008-0160423A1, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to photolithographic techniques for use infabricating integrated circuits.

2. Description of the Related Art

Photolithography is a photographic process used to transfer circuitpatterns onto a semiconductor wafer. The process generally involvesprojecting light through a patterned mask onto a silicon wafer which iscovered with a photosensitive film, e.g., photoresist. A stepper moveacross the mask, projecting light through the mask to image portions ofthe photoresist corresponding to the mask pattern. The wafer is thendeveloped so that the exposed or unexposed portions of the photoresistare removed, depending on the type of resist used. Whilephotolithography is a relatively mature technology, new challenges areencountered by the ever present desire to scale down feature sizes.

SUMMARY OF THE INVENTION

The present invention addresses the above and other issues by providinga photolithographic method for fabricating a pattern on a substrateusing different exposure patterns.

In one embodiment, a photolithographic method for fabricating a patternon a photosensitive layer on a substrate includes comprises firstexposing of the photosensitive layer using a mask and hyper-numericalaperture optics providing a first exposure pattern, and second exposingof the photosensitive layer using the mask and hyper-numerical apertureoptics providing a second exposure pattern, different than the firstexposure pattern. Thus, the same mask is used for both exposures.Further, the photosensitive layer remains in a fixed position relativeto the mask during the first and second exposing.

A hyper-numerical aperture is an aperture greater than one, and may beachieved using water immersion optics, for instance. In one approach,the first and second exposure patterns comprise respective orthogonaldipole exposure patterns, e.g., an x-dipole pattern and a y-dipolepattern.

Further, the first exposing results in a light intensity pattern on thephotosensitive layer in which lines of intensity minima aresubstantially equally spaced in a first direction, and the secondexposing results in a light intensity pattern on the photosensitivelayer in which lines of intensity minima are substantially equallyspaced in a second direction, orthogonal to the first direction. Forexample, the lines of intensity minima in the first direction can besubstantially equally spaced at a pitch of approximately 70-150 nm, andthe lines of intensity minima in the second direction can besubstantially equally spaced in the second direction also at a pitch ofapproximately 70-150 nm. Spacing of the lines of intensity minima in thefirst direction can be substantially equal to, or can differ from,spacing of the lines of intensity minima in the second direction.

The method may further include developing the photosensitive layer toform a mask from the photosensitive layer, and etching a layer which isbeneath the photosensitive layer using the mask formed from thephotosensitive layer, thereby forming a two-dimensional array offeatures, such as posts, in the layer which is beneath thephotosensitive layer.

The developing can be performed after the first and second exposingwithout performing additional exposing of the photosensitive layer afterthe first and second exposing. For example, no additional exposure usinga trim mask need be performed.

In another embodiment, a photolithographic method for fabricating apattern on a photosensitive layer on a substrate includes first exposingof the photosensitive layer using a mask and optics providing a firstexposure pattern, resulting in a light intensity pattern on thephotosensitive layer in which lines of intensity minima aresubstantially equally spaced in a first direction at a pitch ofapproximately 70-150 nm, and second exposing of the photosensitive layerusing the mask and optics providing a second exposure pattern, differentthan the first exposure pattern, resulting in a light intensity patternon the photosensitive layer in which lines of intensity minima aresubstantially equally spaced in a second direction, orthogonal to thefirst direction, at a pitch of approximately 70-150 nm. For instance,the second exposure pattern can be orthogonal to the first exposurepattern.

In another embodiment, a photolithographic method for fabricating apattern on a photosensitive layer on a substrate includes first maskedexposing of the photosensitive layer using optics providing a firstexposure pattern, first hard baking of the photosensitive layer afterthe first exposing, and, after the first hard baking, second maskedexposing of the photosensitive layer using optics providing a secondexposure pattern, different than the first exposure pattern. The methodmay further include second hard baking of the photosensitive layer afterthe second exposing, and developing the photosensitive layer after thesecond hard baking.

In another embodiment, a photolithographic method for fabricating apattern on a photosensitive layer on a substrate includes transferring afirst mask pattern to a photosensitive layer, where the first maskpattern includes spaced apart rows which extend in a first direction.The method may further include performing a first etching operation on ahard mask layer beneath the photosensitive layer, after transferring thefirst mask pattern, and transferring a second mask pattern to thephotosensitive layer, where the second mask pattern includes spacedapart rows which extend in a second direction. The method may furtherinclude performing a second etching operation on the hard mask layer,after transferring the second mask pattern and etching a layer which isbeneath remaining portions of the hard mask layer. Further, a strippingoperation may be performed on the photosensitive layer, after performingthe second etching operation, thereby revealing the remaining portionsof the hard mask layer.

The first and second mask patterns can be transferred to thephotosensitive layer via a common mask, by rotating the mask ninetydegrees, or by rotating the photosensitive layer ninety degrees, forinstance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a photolithographic apparatus.

FIG. 2 depicts a profile of an x-dipole illuminator.

FIG. 3 depicts lines of intensity maxima and minima on a photoresistafter exposure using the x-dipole illuminator of FIG. 2.

FIG. 4 depicts a profile of a y-dipole illuminator.

FIG. 5 depicts lines of intensity maxima and minima on a photoresistafter exposure using the y-dipole illuminator of FIG. 4.

FIG. 6 depicts areas of intensity maxima and minima on a photoresistafter exposure using the x-dipole illuminator of FIG. 2 and the y-dipoleilluminator of FIG. 4.

FIG. 7 depicts a top view of the poly layer which is revealed afterdeveloping the photoresist of FIG. 6.

FIG. 8 depicts a tilted view of post structures formed after etching thepoly later beneath a positive photoresist such as shown in FIG. 7.

FIG. 9 depicts a tilted view of voids formed after etching a poly laterbeneath a negative photoresist such as shown in FIG. 7.

FIG. 10 depicts a photolithographic process which includes first andsecond exposures of a photo-sensitive layer via a common mask usingfirst and second optics having different exposure patterns.

FIG. 11 depicts a photolithographic process which includes first andsecond hard bakes of a photo-sensitive layer after first and secondexposures, respectively, using optics having different exposurepatterns.

FIG. 12 depicts a cross-sectional view of a substrate including poly,hard mask and photoresist layers.

FIG. 13 depicts a top view of the substrate of FIG. 12 with a firstexposure pattern.

FIG. 14 depicts a cross-sectional view of the substrate of FIG. 13showing exposed and unexposed rows of the photoresist.

FIG. 15 depicts the substrate of FIG. 14 after developing thephotoresist to form a mask.

FIG. 16 a depicts a top view of FIG. 15.

FIG. 16 b depicts a top view of the substrate of FIG. 16 a after etchingthe hard mask using the photoresist mask.

FIG. 17 depicts a cross-sectional view of the substrate of FIG. 16 bshowing the hard mask etched down to the poly layer.

FIG. 18 depicts a top view of the substrate of FIG. 17 with a secondexposure pattern.

FIG. 19 depicts the substrate of FIG. 18 after developing thephotoresist to form a mask.

FIG. 20 depicts the substrate of FIG. 19 after etching portions of thehard mask down to the poly layer using the photoresist mask.

FIG. 21 depicts the substrate of FIG. 20 after stripping the remainingphotoresist, revealing remaining portions of the hard mask.

FIG. 22 depicts a cross-sectional view of the substrate of FIG. 21showing etching of the poly layer using the remaining portions of thehard mask.

FIG. 23 depicts a photolithographic process based on FIGS. 12-22.

DETAILED DESCRIPTION

The present invention provides a photolithographic method forfabricating a pattern on a substrate using different exposure patterns.

FIG. 1 depicts a photolithographic apparatus. The apparatus, showngenerally at 100, includes a control 110, stepper motor 120, lightsource 130, optical element 140, mask 150, aperture 160 and projectionlens 170. A pattern of the mask 150 is transferred to a photoresist film180 on a substrate 190 such as a wafer. A polysilicon (poly) layer 185is provided on the substrate in an example implementation. Othermaterials can be used instead of poly in a layer beneath the photoresistfilm 180, such as tungsten or other metal substrate. The photoresistfilm has a pre-determined thickness which is suitable for its intendedapplication. In this simplified example, under control of the steppermotor 120, the light source 130 and optical element 140 move relative tothe mask 150, while the photoresist film 180 is held in a fixed positionin relation to the mask 150. In particular, the light source 130 andoptical element 140 expose portions of the photoresist film 180 as theymove across the pattern of the mask 150. Further, the optical element140 can be configured to use at least first and second optical elementshaving different exposure patterns. For example, a first exposure of thephotoresist film 180 can be performed with the first optical element inplace, after which a second exposure of the photoresist film 180 can beperformed with the second optical element in place.

In one possible approach, the first and second optics can have exposurepatterns which are orthogonal to one another. This can be achieved,e.g., using an x-dipole illuminator 142 and a y-dipole illuminator 144which are used at different times for the first and second exposures. Inone possible approach, the optical element 140 is held in a rotating orotherwise movable holder which holds the x-dipole illuminator 142 and ay-dipole illuminator 144, and can be moved to select either the x-dipoleilluminator 142 or the y-dipole illuminator 144 for exposing thephotoresist film 180. The lens 170 provides reduction optics whichreduces the incident light beam to cause an exposure pattern on thephotoresist film corresponding to a pattern on the mask 150. The mask150 can be a chrome-less mask, chrome-on-glass mask or attenuating phaseshifting mask, for example. The mask can be provided with a mask biasand phase angle which is optimum for its intended application.

Further, the photolithographic apparatus 100 may use immersionlithography. In one approach, water is dispensed between the lens 170and the photoresist film 180, and surface tension results in a puddle onthe photoresist film 180. Since the index of refraction (n) of water isn>1 (e.g., 1.47 for ultrapure water), a numerical aperture (NA) of >1can be achieved. The lens 170 in combination with the water puddleprovide hyper-numerical aperture (NA>1) optics which can resolve asmaller feature width than the lens 170 in air. For example, with thelight source 130 providing UV light at λ=193 nm, and NA=1.2, a featuresize of about 45 nm can be achieved. Specifically, with the relation:feature size=k1xλ/NA, we have 45 nm=0.28×193 nm/1.2. An NA ofapproximately 1.0-1.5 may be used, for instance.

In one possible implementation, the lithographic apparatus 100 can beused to image 45 nm post structures, also referred to as pillars, in thepoly layer 185 beneath the photoresist 180, using the pattern formed inthe photoresist as a mask. Such structures are useful in providingdifferent types of integrated circuits including those used forproviding non-volatile memory. However, imaging post structures to meetdesired feature size criteria can be extremely challenging. Further,improved resolution can be achieved by imaging the photoresist film 180in multiple exposures, including a first exposure which uses firstoptics providing a first exposure pattern followed by a second exposurewhich uses second optics providing a second exposure pattern. In oneapproach, the exposure patterns are orthogonal to one another. In aspecific implementation, the first optics includes the x-dipoleilluminator 142 and the second optics includes the y-dipole illuminator144, as discussed further below.

FIG. 2 depicts a profile of an x-dipole illuminator. The profile isdepicted by reference to an x-y axis, where x and y positions extendsbetween respective normalized indices of −1 and 1. The x-dipoleilluminator 200 includes an opaque region 210 and two opposing apertures215 and 220 through which light passes for exposing the photoresist film180.

FIG. 3 depicts lines of intensity maxima and minima on a photoresistafter exposure using the x-dipole illuminator of FIG. 2. The maskpattern 152 as projected on the photoresist 180 is depicted, in additionto lines of intensity maxima 310 and lines of intensity minima 320 whichextend across a plane of the photoresist film 180 in a vertical lineimage. The lines of intensity maxima 310 represent regions on thephotoresist film 180 which are exposed to a maximum light intensity fromthe light source 130, while the lines of intensity minima 320 representregions on the photoresist film 180 which are exposed to a minimum lightintensity from the light source 130. Essentially, the intensity patterninclude areas of substantially equal intensity in the y direction, for agiven x position, while for a given y position, the intensity modulatesperiodically from a minimum to a maximum in the x direction.

Further, the lines of intensity maxima 310 can be substantially equallyspaced apart from one another in the x direction, while the lines ofintensity minima 320 can also be substantially equally spaced apart fromone another in the x direction. Furthermore, the lines of intensitymaxima 310 and the lines of intensity minima 320 can also besubstantially equally spaced apart from one another in the x direction.

FIG. 4 depicts a profile of a y-dipole illuminator. The profile isdepicted by reference to an x-y axis, where x and y positions extendsbetween respective normalized indices of −1 and 1. The y-dipoleilluminator 400 includes an opaque region 410 and two opposing apertures415 and 420 through which light passes for exposing the photoresist film180.

The y-dipole illuminator can be the same as the x-dipole illuminator 200rotated by 90 degrees in one possible approach. In particular, thisapproach results in an exposure pattern which is similar in the x and ydirection. As a result, the structures formed in the photoresist filmafter developing will have a symmetric shape in the x and y directions.In another approach, the y-dipole illuminator, when rotated by 90degrees, differs from the x-dipole illuminator 200. For example, thesize of the illuminator, and/or the size and/or position of theapertures 415 and 420 can differ. To illustrate, reducing the length ofthe apertures 415 and 420 in the y direction will result in higher lightintensity profile.

FIG. 5 depicts lines of intensity maxima and minima on a photoresistafter exposure using the y-dipole illuminator of FIG. 4. The maskpattern 152 as projected on the photoresist 180 is depicted, in additionto lines of intensity maxima 510 and lines of intensity minima 520 whichextend across a plane of the photoresist film 180 in a horizontal lineimage. The lines of intensity maxima 510 represent regions on thephotoresist film 180 which are exposed to a maximum light intensity fromthe light source 130, while the lines of intensity minima 520 representregions on the photoresist film 180 which are exposed to a minimum lightintensity from the light source 130. Essentially, the intensity patterninclude areas of substantially equal intensity in the x direction, for agiven y position, while for a given x position, the intensity modulatesperiodically from a minimum to a maximum in the y direction.

Further, the lines of intensity maxima 510 can be substantially equallyspaced apart from one another in the y direction, while the lines ofintensity minima 520 can also be substantially equally spaced apart fromone another in the y direction. Furthermore, the lines of intensitymaxima 510 and the lines of intensity minima 520 can also besubstantially equally spaced apart from one another in the y direction.

Additionally, referring to FIGS. 3 and 5, depending on the illuminatorswhich are used, the spacing of the lines of intensity maxima 310 and 510can be substantially equally to one another, and/or the lines ofintensity minima 320 and 520 can be substantially equally to oneanother. Also, as mentioned, if the illuminators 200 and 400 differbeyond the 90 degree rotation, the spacing of the lines of intensitymaxima 310 in the x direction can be substantially different than thespacing of the lines of intensity maxima 510 in the y direction.Further, the spacing of the lines of intensity minima 320 in the xdirection can be substantially different than the spacing of the linesof intensity minima 520 in the y direction. Structures whosecross-section in a plane of the photoresist is generally square shapedor rectangular shaped (with adjacent sides of different lengths) can beformed depending on the spacing of the lines of minima or maxima in thex and y directions.

FIG. 6 depicts areas of intensity maxima and minima on a photoresistafter exposure using the x-dipole illuminator of FIG. 2 and the y-dipoleilluminator of FIG. 4. Exposure using the two illuminators results in anintensity pattern on the photoresist film 180 which is represented by a2-D grid in which the lines of intensity maxima 310 in the x directionintersect with the lines of intensity maxima 510 in the y direction, atareas of intensity maxima 610, depicted as generally square areas.Further, the lines of intensity minima 320 in the x direction intersectwith the lines of intensity minima 520 in the y direction, at areas ofintensity minima 620, depicted as generally square areas. In practice,the square shape is rounded off at the edges. After the photoresist film180 is developed to form a mask, the poly beneath the photoresist maskis etched so that features such as post structures or voids 720 (FIG. 7)are formed in the poly beneath the areas of intensity minima 620,depending on whether the photoresist film 180 is a positive or negativephotoresist, respectively. FIG. 7 depicts a top view of the poly layerwhich is revealed after developing the photoresist of FIG. 6, and theremaining photoresist portions which are square in this example.

FIG. 8 depicts a tilted view of post structures formed after etching thepoly later beneath a positive photoresist such as shown in FIG. 7. Ifthe photoresist film 180 is a positive photoresist, indicating thatareas exposed to maximum intensity are removed and areas exposed tominimum intensity remain, features such as post structures 810 can beformed in the poly layer 185 or other material which is beneath thephotoresist film by etching the poly layer using the pattern provided bythe remaining photoresist. The features can be spaced apart by a pitchof approximately 70-150 nm, for instance. In an example embodiment, thepitch is approximately 90 nm.

FIG. 9 depicts a tilted view of voids formed after etching a poly laterbeneath a negative photoresist such as shown in FIG. 7. If thephotoresist film 180 is a negative photoresist, indicating that areasexposed to maximum intensity remain and areas exposed to minimumintensity are removed, voids 900 are formed. For instance, voids such asholes for dense array contact hole printing can be provided.

FIG. 10 depicts a photolithographic process which includes first andsecond exposures of a photo-sensitive layer via a common mask usingfirst and second optics having different exposure patterns. Note that inthis and the other flowcharts, not all necessary steps are shown.Photolithography generally involves a number of steps, including surfacepreparation, which can include wafer cleaning and priming, coating thewafer with the photoresist such as by spin coating, and a pre-exposurebake (soft bake) which is used to evaporate the coating solvent and todensify the resist after spin coating. Other steps include alignment ofthe mask to the substrate, exposure of the photoresist, post-exposurebake of the photoresist, and development of the photoresist in which thephotoresist is washed in a development solution which removes exposedareas of the photoresist (for a positive photoresist) or unexposed areasof the photoresist (for a negative photoresist). A post-exposure bake isused to activate a chemically amplified reaction in the exposed area. Apost-development hard bake is used to stabilize and harden the developedphotoresist, after which processing using the photoresist as a maskingfilm is performed to transfer the pattern of the mask to the substratebelow the photoresist. Other steps include stripping the photoresistfrom the substrate and post processing cleaning.

A double exposure photolithographic process using a single mask issuitable for imaging features such as post structure of approximately 45nm, in one implementation. For instance, x and y dipole exposuresprovide a large process window and can reduce optical interference,e.g., compared to C-quad (quadrupole) illumination. Further, in onepossible approach, a chrome-less mask with optimal mask bias and phaseangle is used. The process includes arranging the substrate with thephotoresist film in fixed relation to a mask (step 1000). At step 1010,first optics having a first exposure pattern, such as the x-dipoleilluminator, are positioned in the photolithography apparatus, e.g., asoptical element 140 in the apparatus 100 of FIG. 1. At step 1020, thephotoresist is exposed using the mask and the first optics in a firstlithographic pass. At step 1030, second optics having a second exposurepattern, such as the y-dipole illuminator, are positioned in thephotolithography apparatus in place of the first optics. At step 1040,the photoresist is exposed using the same mask and the second optics ina second lithographic pass. At step 1050, a post-exposure hard bake isperformed on the wafer and, at step 1060, developing to form a mask fromthe photoresist is performed. Typically, the wafer is removed from thephotolithography apparatus after being exposed, then placed in an ovenfor the post-exposure hard bake. The wafer is then removed from the ovento another area for developing the photoresist. Specific parameters ofthe hard bake, such as time and temperature, can be set according to theapplication and layers used. Additionally, specific parameters of thedeveloping can be set according to the application and layers used. Step1070 depicts etching of the poly layer via the photoresist mask to formfeatures such as posts or other raised structures or voids in the polylayer. A 2-D array of such features can be formed in a plane of thepoly.

FIG. 11 depicts a photolithographic process which includes first andsecond hard bakes of a photo-sensitive layer after first and secondexposures, respectively, using optics having different exposurepatterns. In one possible approach, a chrome-less mask with optimal maskbias and phase angle is used. The process includes arranging thesubstrate with the photoresist film in fixed relation to a mask (step1100). At step 1110, first optics having a first exposure pattern, suchas the x-dipole illuminator, are positioned in the photolithographyapparatus. At step 1120, the photoresist is exposed using the mask andthe first optics in a first lithographic pass. At step 1130, a firstpost-exposure hard bake is performed. The first post-exposure bakeserves to erase a “memory effect” of a photon distribution which occursin the photoresist due to the first exposure. The photon distribution inthe photoresist after the first exposure would otherwise persist and bealtered by the second exposure, reducing the sharpness of the lightintensity and also reducing the process window. Further, parameters ofthe first and second post exposure bakes can be set to an optimal levelbased on the specific implementation. In one example implementation, thetime and/or temperature of the first post exposure bake can beapproximately 5 to 15% higher than in the second exposure bake.

The wafer can be removed from the photolithography apparatus and placedin an oven for the hard bake, which is also used to stabilize and hardenthe photoresist. After the hard bake, and subsequent cooling, the waferis returned to the photolithography apparatus and re-aligned to themask. The same mask or a different mask can be used. At step 1140,second optics having a second exposure pattern, such as the y-dipoleilluminator, are positioned in the photolithography apparatus in placeof the first optics. At step 1150, the photoresist is exposed using themask and the second optics in a second lithographic pass. At step 1060,the wafer is removed from the photolithography apparatus and placed inan oven for a second post-exposure hard bake. In one exampleimplementation, as mentioned, the time and/or temperature of the secondpost exposure bake can be approximately 5 to 15% lower than the firstexposure bake. The wafer is then removed from the oven to another areafor developing the photoresist to form a mask from the photoresist, atstep 1070. Specific parameters of the hard bakes can be set according tothe application and layers used. Additionally, specific parameters ofthe developing can be set according to the application and layers used.Step 1080 depicts etching of the poly layer via the photoresist mask toform features such as posts or other raised structures or voids in thepoly layer. A 2-D array of such features can be formed in a plane of thepoly or other material beneath the photoresist.

FIGS. 12-23 describe an additional photolithographic process which usesa hard mask layer on the substrate for patterning desired features in apoly layer or other material below the hard mask layer. In particular,FIG. 12 depicts a cross-sectional view of a substrate 1240 includingpoly 1230, hard mask 1220 and photoresist layers 1210. An x-direction isdepicted. A y-direction extends orthogonal to the x-direction, into thepage.

FIG. 13 depicts a top view of the substrate of FIG. 12 with a firstexposure pattern. A mask having a pattern represented by spaced apartrows can be used to provide exposed rows, such as example exposed row1310, and unexposed rows, such as example unexposed row 1320, extendingin the y-direction, for example, on the photoresist 1210. FIG. 14depicts a cross-sectional view of the substrate of FIG. 13 showing thealternating exposed rows, e.g., row 1310 and the unexposed rows, e.g.,row 1320, of the photoresist.

FIG. 15 depicts the substrate of FIG. 14 after developing thephotoresist to form a mask. In this example, the photoresist is apositive photoresist, so the exposed portions of the photoresist areremoved, leaving the unexposed rows, e.g., row 1320, of the photoresistand revealing rows of the hard mask 1610 where the photoresist isremoved. FIG. 16 a depicts a top view of FIG. 15.

FIG. 16 b depicts a top view of the substrate of FIG. 15 after etchingthe hard mask using the photoresist mask, thereby revealing rows 1620 ofthe poly layer or other material beneath the hard mask. Thus, spacedapart rows of the hard mask which are revealed when the photoresist isdeveloped are etched down to the poly to provide the configuration ofFIG. 16 b. FIG. 17 depicts a cross-sectional view of the substrate ofFIG. 16 b showing portions of the hard mask 1220 etched down to the polylayer 1230.

FIG. 18 depicts a top view of the substrate of FIG. 17 with a secondexposure pattern. A mask having a pattern represented by spaced apartrows can be used to provide exposed rows, such as example exposed row1810, and unexposed rows, such as example unexposed row 1820, extendingin the x-direction, for example, on the photoresist. For example, thesame mask may be used as in FIG. 13 but rotated ninety degrees. Or, thesubstrate may be rotated ninety degrees relative to the mask. Thus, thespaced apart rows of the two patterns may extend in orthogonaldirections. FIG. 19 depicts the substrate of FIG. 18 after developingthe photoresist to form a further photoresist mask. Portions of the hardmask 1930 are revealed when the photoresist is developed this secondtime. Here, the remaining portions of the photoresist are squares, suchas example square 1920, although other shapes such as rectangles can beprovided as well. Specifically, the remaining portions of thephotoresist can be rectangular, with adjacent sides of unequal length,if the rows in the x and y direction have different widths, forinstance. The revealed rows 1620 of the poly are also indicated. FIG. 20depicts the substrate of FIG. 19 after etching portions of the hard mask1930 down to the poly layer or other layer beneath the photoresist usingthe photoresist mask, revealing the poly layer 2010.

FIG. 21 depicts the substrate of FIG. 20 after stripping the remainingphotoresist, revealing remaining portions of the hard mask. Here, thephotoresist squares, such as example square 1920 are stripped away toreveal corresponding portions of the hard mask beneath the strippedphotoresist, such as example hard mask square 2120. As mentioned, thesquare shape is provided as an example only, as other shapes may beformed. The hard mask squares extend in a 2-D array across thesubstrate.

FIG. 22 depicts a cross-sectional view of the substrate of FIG. 21showing etching of the poly layer or other layer using the remainingportions of the hard mask, such as the hard mask squares 2120.Essentially, the pattern formed by the remaining hard mask squares istransferred to the poly layer beneath the hard mask layer, so that a 2-Darray of features is formed. The hard mask may or may not removed fromthe poly or other layer depending on the type of hard mask used and typeof etch process used. For example, the features may be raised structuressuch as shown in FIG. 8. Alternatively, the features may be voids, e.g.,holes, such as when a negative photoresist is used.

FIG. 23 depicts a photolithographic process based on FIGS. 12-22.Generally, an example process includes transferring a first mask patternto a photosensitive layer, where the first mask pattern includes spacedapart rows which extend in a first direction, and performing a firstetching operation on a hard mask layer beneath the photosensitive layer,after transferring the first mask pattern. A second mask pattern is thentransferred to the photosensitive layer, where the second mask patternincludes spaced apart rows which extend in a second direction, such asorthogonal to the first direction. A second etching operation isperformed on the hard mask layer after transferring the second maskpattern. A layer, such as a poly layer, which is beneath the remainingportions of the hard mask layer, is then etched to form desired featuresin the poly layer.

For example, step 2300 includes exposing rows of photoresist in the ydirection using a first mask pattern, step 2310 includes developing thephotoresist to form a first photoresist mask, which has spaced apartrows corresponding to the first mask pattern (see, e.g., FIGS. 13 and14). Step 2320 includes etching rows of the hard mask in the y directionusing the first photoresist mask, revealing the poly underneath (see,e.g., FIGS. 16 b and 17). Step 2330 includes exposing rows of thephotoresist in the x direction (see, e.g., FIG. 18). Step 2340 includesdeveloping the photoresist again to form a second mask from thephotoresist (see, e.g., FIG. 19). Step 2350 includes etching the hardmask using the second photoresist mask in the x direction (see, e.g.,FIG. 20). Step 2360 includes stripping the remaining photoresist,revealing remaining portions of the hard mask (see, e.g., FIG. 21). Step2370 includes etching the poly beneath the squares of the hard mask toform features such as posts (see, e.g., FIGS. 8 and 22).

Thus, the first and second mask patterns can be transferred to thephotosensitive layer via a common mask, e.g., by rotating the maskninety degrees, or by rotating the photosensitive layer ninety degrees,for instance. Generally, the common mask can be in a first relativeposition with respect to the photosensitive layer for transferring thefirst mask pattern, and in a second relative position with respect tothe photosensitive layer for transferring the second mask pattern, wherethe first relative position is rotated relative to the second relativeposition. Or, the first and second mask patterns can be transferred tothe photosensitive layer via different masks. Moreover, additionalpatterns can be transferred to the photosensitive layer as well.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A photolithographic method for fabricating a pattern on aphotosensitive layer on a substrate, comprising: transferring a firstmask pattern to a photosensitive layer, the first mask patterncomprising spaced apart rows which extend in a first direction;performing a first etching operation on a hard mask layer beneath thephotosensitive layer, after transferring the first mask pattern;transferring a second mask pattern to the photosensitive layer, thesecond mask pattern comprising spaced apart rows which extend in asecond direction; performing a second etching operation on the hard masklayer, after transferring the second mask pattern; and etching a layerwhich is beneath remaining portions of the hard mask layer.
 2. Thephotolithographic method of claim 1 further comprising: performing astripping operation on the photosensitive layer, after performing thesecond etching operation, thereby revealing the remaining portions ofthe hard mask layer.
 3. The photolithographic method of claim 1,wherein: the first direction is orthogonal to the second direction. 4.The photolithographic method of claim 1, wherein: the first and secondmask patterns are transferred to the photosensitive layer via a commonmask, the common mask is in a first relative position with respect tothe photosensitive layer for transferring the first mask pattern, and ina second relative position with respect to the photosensitive layer fortransferring the second mask pattern, the first relative position isrotated relative to the second relative position.
 5. Thephotolithographic method of claim 1, wherein: the etching of the layerwhich is beneath the remaining portions of the photosensitive layerresults in a two-dimensional array of features in the layer which isbeneath the remaining portions of the photosensitive layer.
 6. Thephotolithographic method of claim 5, wherein: the features compriseposts.